New Idea for Winning condition...
Right now, the two card types are Monster and Spell cards. We can have a third card type called Heros... Each deck has 3 Heros with a certain amount of HP and an effect. If you have no monsters on the field, your opponent can attack your hero, and your hero cannot heal during Recharges. Also, Heros are unaffected by all card effects. This way, you can deside the winner by destroying all the hero cards... I will make an example of a hero in a second.